Solar thermal technology concentrates solar radiation by using mirrors (also referred to as “heliostats”) to direct solar radiation to a central tower receiver. In the central tower receiver (also referred to as the “central solar receiver”), solar energy is absorbed as heat and converted into electricity. Within the central tower receiver are heat exchangers for transferring heat to a heat transfer fluid, which is then used to produce steam to run a turbine and extract work to produce electricity. Considering the amount of energy directed to the central solar receiver, materials for the central solar receiver must be able to withstand operating temperatures of 700° C. or more and still have high absorptivity. To improve the stability of the materials at high temperatures and the absorptivity of the materials, coatings have been applied to components of the central solar receiver, such as the heat exchanger tubes.
Coatings for solar receiver heat exchangers can greatly impact the performance of a solar plant as the coating can greatly affect the efficiency of the solar energy absorption. However, while such coatings may have a high absorption, the coatings are typically not oxidation resistant. Once exposed to high temperatures, oxidation of the heat exchanger tubes results in oxidation products that interfere with the performance of the heat exchanger. For example, an oxide layer formed at the interface between the heat exchanger substrate and coating may reduce the mechanical properties and thermal heat transfer of the system. For instance, polymer based coatings that are to be vitrified after post-heat treatment do not provide oxidation resistance. Typically, vitrification does not take place completely, leaving the coating with low oxidation resistance. The oxidation can lead to cracks in the coating and, over time, increase the difference in the thermal expansion of the coating and the heat exchanger materials leading to spallation of the coating.
Accordingly, there remains a need for improved coatings for heat exchanger tubes and methods of making such coatings to increase absorption of solar radiation while also decreasing oxidation of the heat exchanger tubes.